I set out to show him how, sticking to his menu while substituting cheaper versions. Once I bought those items, I was going to do a second round of shopping with my spare money to buy him a few extras — eggs, a little brick of cheese, some store-brand vinegar for salad dressing and maybe an onion or two. (The mayor is a vegetarian but eats dairy products.)

So I chose heads of romaine lettuce instead of bagged salad; canned yams instead of fresh; and store-brand beans and corn instead of the Goya brand. I was feeling pretty smug.

The mayor’s total was $29.78 — just pennies under his theoretical weekly budget of $30.09. I figured I’d shave a good $10 off that, thanks in part to his olive oil misstep.

Surprise, surprise: My frugal hausfrau total was $29.60. The only additional item I could afford was a 48-cent onion.

Booker shopped at the Pathmark in Irvington because it was within walking distance of his home. (Many on food stamps have no way to get to stores beyond their neighborhood.)

Accompanying him was Elizabeth Reynoso, the city’s food policy director. She says they shopped carefully, comparing the unit prices before making choices. The mayor planned to build his meals around potatoes, but when he saw sweet potatoes were on sale — half off! — he switched to them.

He decided they’d be his main carbohydrate, not pasta or rice. "We looked at rice and he said, ‘I can’t afford it — I really can’t,’ " she said.

They compared the price of dried beans with the cans — also on sale — and the cans were actually cheaper.

She says she tried to dissuade him from buying bagged salad, but he was wooed by the discounted two-for-$5 price. He also went for the three-for-$5 frozen chopped broccoli and cauliflower mix. In short, he shopped the sales.

Booker's choices.Tim Farrell/The Star-Ledger

As for that fancy-schmancy olive oil, it was deeply discounted the day they shopped. Booker selected it because it was on sale and a small enough size to fit into his budget, she said, not because it was organic.

"He didn’t really care. It was price, price, price. Plus, that was about the amount of money he had left," she said. (Even then, bargain-brand olive oil like the kind I bought ate up $3.99; anyone really on food stamps would be forced to use store-brand cooking oil instead.)

Booker shopped by price and personal preference, not by calories, so Reynoso isn’t sure if his grocery haul provides enough calories — 2,600 a day — for a man his age.

When you have to shop week to week, paycheck to paycheck, you are stuck with a paradox: The larger cans and boxes are a better value — but you can afford only the smaller versions. Economies of scale are beyond your reach.

He said he regretted he failed to use coupons or to buy eggs. If he kept it up, week after week, he’d learn. He’d snag bargains from the day-old bread bin or the dented-can shelf. He’d carry around the store circular while he shopped, hunting down the weekly specials. He’d walk right by all the olive oil without giving it a second look.